When abnormal extra charging (overcharging) is generated while a secondary battery is charged, a voltage of the battery exceeds a normal range and the battery becomes an instable source of energy.
In such cases, an electrolytic solution, which generates gas, is decomposed inside the battery. The generated gas which is flammable gas has an extremely high ignition risk.
Meanwhile, with the recent demand for a high energy battery, the size and capacity of secondary batteries have been increased correspondingly. As a result, when a secondary battery with increased size and capacity is overcharged, a technology to lower the ignition risk is required.